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Abstract:

A compact monostatic optical transmitter/receiver device simultaneously
transmits an optical beam and collects returning light using a single
lens or optical aperture. The system provides automatic alignment of the
transmit and receive aperture and is compatible with fiber-coupled laser
sources. Transmit light is emitted from a double-cladding fiber core
while received light is coupled into the inner cladding of the same
fiber. The transmit light propagating in the core and the received light
propagating in the inner cladding are separated by the means of a
diplexer comprised of a fused fiber coupler or a fiber-coupled
micro-optic device.

Claims:

1. A monostatic laser system, comprising: a photodetector fiber coupled
to receive a received light; a light source fiber coupled to transmit a
transmitted light; a diplexer fiber coupled to the photodetector to
separate the received light, fiber coupled to the light source to
separate the transmitted light, and fiber coupled on an output side of
the diplexer; and a transmit/receive lens having one side facing a
flat-polished end of said fiber coupled on said output side of the
diplexer.

2. The system according to claim 1, wherein said monostatic laser system
is a monostatic laser detection and ranging system for laser range
finding.

3. The system recited in claim 1, wherein the light source is based on a
fiber master-oscillator/power-amplifier device operating at near 1550 nm,
or 1060 nm, seeded with pulse modulated laser diode.

4. The system recited in claim 1, wherein said diplexer is based on a
fused fiber coupler or a fiber-coupled micro-lens structure.

5. The system recited in claim 1, wherein said fiber coupled on the
output side of the diplexer is based on a double-cladding fiber
comprising an outer cladding, an inner cladding and a core.

6. The system recited in claim 1, wherein the transmitted light emerges
from the flat-polished end of said fiber coupled on said output side of
the diplexer towards the one side of said transmit/receive lens, and is
collimated by said transmit/receive lens, wherein a focal length of said
lens is chosen to yield a desired collimated beam diameter and divergence
angle.

7. The system recited in claim 5, wherein said core has a small diameter
in the range of 5 to 20 μm with a numerical aperture range of 0.08 to
0.16, and wherein said inner cladding has a large diameter range of about
50 to 500 μM and a higher numerical aperture range of about 0.4 to
0.7.

8. The system recited in claim 1, wherein a numerical aperture of the
lens is chosen to be equal to a numerical aperture of a cladding of said
fiber coupled on the output side of the diplexer, which is substantially
larger than a numerical aperture of a core of said fiber coupled on the
output side of the diplexer, thereby a central portion of the lens is
used to collimate the transmitted light.

9. The system recited in claim 1, wherein light reflected from a remote
diffused surface is collected by the lens and focused onto the
flat-polished end of said fiber coupled on said output side of the
diplexer.

10. The system recited in claim 9, wherein of said light focused onto the
flat-polished end of said fiber, only a portion within a numerical
aperture a core of said fiber is launched into the core, another portion
being guided by a cladding of said fiber, wherein said inner core and
said cladding are concentric for ease of alignment with respect to a
focal plane of the lens.

11. The system recited in claim 10, wherein for a typical inner cladding
numerical aperture of about 0.47 and a core numerical aperture of about
0.12, more than 93% of light collected by the lens can be coupled into
the cladding.

12. The system recited in claim 1, wherein the lens surface is shaped so
that it performs near a diffraction limit over a small fraction of its
diameter but operates far from diffraction limit over the remaining
surface.

13. The system recited in claim 1, wherein a lens with a numerical
aperture of about 0.5 produces near-diffraction limited beam collimation
for light emerging from a core of said fiber coupled on said output side
of the diplexer, wherein a numerical aperture of said core is about 0.12,
producing a focused spot that is significantly larger than diffraction
limit for collected light filling the entire lens surface corresponding
to a numerical aperture of 0.5.

14. An optical diplexer based on a fused fiber coupler to separate a
received light propagating in a receive direction from a transmit light
that is propagating in a transmit direction, and function as a
fiber-based monostatic transmitter/receiver, said diplexer comprising: a
double-cladding fiber having a core for coupling at one end to an optical
transmitter source; and a multimode-core fiber having a single cladding
for coupling at one end to a receiver photodiode, the double-cladding
fiber and the multimode-core fiber being fused together to be in optical
contact with each other in a coupling region to allow optical power
transfer between an inner cladding of the double-cladding fiber and the
multimode-core fiber, wherein an outer cladding of the double-cladding
fiber and a jacket of the multimode-core fiber are stripped away in the
coupling region to allow optical contact or fusion of the inner cladding
of the double-cladding fiber and the multimode-core fiber.

15. The optical diplexer recited in claim 14, wherein an optical coupling
is achieved between the inner cladding of the double-cladding fiber and
the multimode-core fiber without causing significant perturbation of
light propagating in the double-cladding fiber core.

16. The optical diplexer recited in claim 14, wherein the optical
transmitter source couples to the core of the double-cladding fiber such
that no significant light loss occurs for the transmit light propagating
through the core of the double cladding.

17. The optical diplexer recited in claim 14, wherein the double-cladding
fiber is gradually tapered down in diameter to achieve a desired
cross-coupling ratio, such that for the single mode core with a diameter
of about 10 μm and numerical aperture of about 0.12, a taper ratio is
below 2.0.

18. A monostatic laser system based on a fiber-coupled micro-lens
structure, comprising: a multimode fiber with its lens face facing a
fiber side of a receiver lens, and a receiver face of the multimode fiber
facing a receiver photodiode; a transmitter fiber having a core with its
lens face facing a fiber side of a transmitter lens, and a transmitter
face of the transmitter fiber facing an optical transmitter source; a
double-cladding fiber having a core with its lens face facing a fiber
side of an output lens, and an output face of the double-cladding fiber
facing an output side; and a diplexer based on an angled mirror
configured in relation to said transmitter lens, receiver lens and output
lens, wherein said transmitter lens and said output lens are arranged in
an imaging configuration, where a near-field intensity distribution from
the core of the transmitter fiber is first imaged by said transmitter
lens at half-point between said transmitter lens and said output lens,
where transmitted light from said optical transmitter source passes
through a hole in said angled mirror.

19. The monostatic laser system recited in claim 18, wherein the
transmitted light is imaged onto the lens face of the double-cladding
fiber by the output lens, wherein if the cores of the transmitter and the
double-cladding fiber are identical, then a high core-to-core coupling
efficiency can be achieved with 1:1 imaging magnification, but for
dissimilar core diameters other magnification factors are used to match
the mode field diameters of the fiber cores.

20. The monostatic laser system recited in claim 18, wherein an image of
a near-field distribution of the double-cladding fiber is imaged onto the
angled mirror by the output lens, reflected and re-imaged onto the lens
face multimode fiber by the receiver lens, thereby a received light
emerging from the inner cladding of the double-cladding fiber is imaged
onto the lens face of the multimode fiber by the output and receiver lens
pair.

Description:

FIELD OF THE INVENTION

[0002] This invention relates to monostatic optical transmitter/receivers
as they relate to laser detection.

BACKGROUND OF THE INVENTION

[0003] A typical method of operation for Laser Detection and Ranging
(LADAR) system or a Laser Rangefinder (LRF) is to emit a short (typically
1-10 ns), high peak power optical pulse in a narrow beam and detect its
reflected return from a target. The time delay between emission and
detection provides the range to the target, and knowledge of the beam
direction provides target bearing information. The transmitted beam is
collimated to provide low divergence to the target so that the incident
spot is sufficiently small to provide required lateral resolution.

[0004] In general, the target will reflectively scatter the incident beam
into a large solid angle, so the amount of reflected power detected at
the LADAR system will be proportional to the area of the receiver
aperture. If the transmitted and received beams share a common aperture
the system is described as monostatic. This type of system avoids
misalignment and parallax problems common to LADARs and LRFs which have
separate transmit and receive apertures (described as bistatic). It makes
manufacturing of LIDARs and LRFs simpler since it eliminates the needed
for time consuming precise alignment of the transmit and receive
apertures required with bistatic systems. In monostatic operation, an
optical means has to be provided for separating the transmitted and
received beams before the aperture, so that the transmitted beam does not
lose energy or degrade the operation of the photodetector, and the
received intensity is directed primarily to the photodetector. In
principle, this optical diplexing function might be as simple as a
beam-splitter (which has excessive round-trip optical loss) or more
complex (such as a non-reciprocal optical circulator).

SUMMARY OF THE INVENTION

[0005] For the exemplary systems described herein, diplexers comprised of
a fused fiber coupler or a fiber-coupled micro-lens structure are
variously used to efficiently separate the transmitted and the received
light.

[0006] In one aspect, a monostatic laser system is disclosed, comprising a
photodetector fiber-coupled to receive a received light; a light source
fiber-coupled to transmit a transmitted light; a diplexer fiber coupled
to the photodetector to separate the received light, fiber-coupled to the
light source to separate the transmitted light, and fiber-coupled on an
output side of the diplexer; and a transmit/receive lens having one side
facing a flat-polished end of said fiber coupled on said output side of
the diplexer.

[0007] In another aspect, an optical diplexer is disclosed based on a
fused fiber coupler to separate a received light propagating in a receive
direction from a transmit light that is propagating in a transmit
direction, and function as a fiber-based monostatic transmitter/receiver.
Such a diplexer comprises a double-cladding fiber having a core for
coupling at one end to an optical transmitter source; and a
multimode-core fiber having a single cladding for coupling at one end to
a receiver photodiode, the double-cladding fiber and the multimode-core
fiber being fused together to be in optical contact with each other in a
coupling region to allow optical power transfer between an inner cladding
of the double-cladding fiber and the multimode-core fiber. An outer
cladding of the double-cladding fiber and a jacket of the multimode-core
fiber are stripped away in the coupling region to allow optical contact
or fusion of the inner cladding of the double-cladding fiber and the
multimode-core fiber.

[0008] Yet, in another aspect, a monostatic laser system is disclosed
based on a fiber-coupled micro-lens structure. Such a monostatic laser
systems comprises a multimode fiber with its lens face facing a fiber
side of a receiver lens, and a receiver face of the multimode fiber
facing a receiver photodiode; a transmitter fiber having a core with its
lens face facing a fiber side of a transmitter lens, and a transmitter
face of the transmitter fiber facing an optical transmitter source; a
double-cladding fiber having a core with its lens face facing a fiber
side of an output lens, and an output face of the double-cladding fiber
facing an output side; and a diplexer based on an angled mirror
configured in relation to said transmitter lens, receiver lens and output
lens. Said transmitter lens and said output lens are arranged in an
imaging configuration, where a near-field intensity distribution from the
core of the transmitter fiber is first imaged by said transmitter lens at
half-point between said transmitter lens and said output lens, where
transmitted light from said optical transmitter source passes through a
hole in said angled mirror.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Additional advantages and features will become apparent as the
subject invention becomes better understood by reference to the following
detailed description when considered in conjunction with the accompanying
drawings wherein:

[0010] FIG. 1 shows a block diagram of an exemplary monostatic Laser
Detection and Ranging (LADAR) system or a Laser Rangefinder (LRF);

[0012] FIG. 3a shows an exemplary diplexer configuration based on a fiber
coupler;

[0013] FIG. 3b shows an exemplary cross-sectional view of a coupling
region of the exemplary diplexer configuration based on a fiber coupler;

[0014] FIG. 4 shows another exemplary diplexer configuration based on a
fiber coupler; and

[0015] FIG. 5 shows an exemplary diplexer based on micro-lenses.

DETAILED DESCRIPTION

[0016] A block diagram of an exemplary monostatic Laser Detection and
Ranging (LADAR) system or a Laser Rangefinder (LRF) is shown in FIG. 1.
As exemplified in FIG. 1, such an exemplary system 100 is comprised of a
photodetector 120 for the received light, a light source 110 for the
transmitted light, and a diplexer 130 to separate the received light from
the transmitted light.

[0017] As disclosed here, the detector 120, light source 110 and diplexer
130 are fiber coupled. The light source 110 can be a fiber
master-oscillator/power-amplifier (MOPA) device operating at near 1550
nm, or 1060 nm, seeded with pulse modulated laser diode. The fiber 140 on
the output (right) side of the diplexer can have a double-cladding
structure. As shown in FIG. 1, a flat-polished end of an exemplary
double-cladding fiber (DCF) 140 faces a transmit/receive lens 150.

[0018] FIG. 2 shows such an exemplary fiber 240 on the output side of the
diplexer having a double-cladding structure comprising an outer cladding
241, an inner cladding 242 and a core 243. FIG. 2 also show the
flat-polished end 244 of the double-cladding fiber (DCF) 240 facing the
TX/RX lens 250.

[0019] An exemplary DCF 240 can be comprised of a small diameter core 243,
typically in the range of dcr=5-20 μm with a numerical aperture
of NAcore=0.08-0.16, and a large diameter dc, higher NA inner
cladding 242, typically dd=50-500 μm and NAclad=0.4-0.7
respectively. The large difference in the NAs of the core and the inner
cladding has been used previously to implement a dual numerical aperture
confocal bar-code scanner.

[0020] Transmitter light emerges from the DCF core 243 and is collimated
by the TX/RX lens 250. The lens focal length f is chosen to give a
desired collimated beam diameter and divergence angle. The lens NA is
chosen to be equal to the NA of the DCF cladding (e.g., 242), which is
substantially larger than DCF core 243 NA, so that only the central
portion of the lens 250 is used to collimate the transmit beam (e.g.,
260). For a specific lens focal length f the field of view (FOV) of the
receiver (e.g., 120) is given by the inner cladding diameter,
FOV=fdc.

[0021] Light reflected (e.g., 270) from a remote diffused surface is
collected by the lens (e.g., 250) and focused onto the flat-polished DCF
end (e.g., 244). Rays focused by the lens (e.g., 250) conic from the full
lens aperture, and their incidence angles cover the full range of the
lens NA. Of those rays that fall on the core region of the DCF only ones
within the NA of the core are launched into the core (e.g., 243), with
the rest captured guided by the high inner DCF cladding (e.g., 242).
Because the inner core (e.g., 243) and its cladding (e.g., 242) are
concentric, at the fiber tip they are automatically aligned with respect
to the common monostatic TX/RX lens (e.g., 250) for any position of the
fiber tip (e.g., 244) in the focal plane of the lens.

[0022] The fraction of received light power (e.g., 270) collected by the
lens (e.g., 250) that is coupled into the inner cladding (e.g., 242) is
given by:

1-(NAcore/NAclad)2

For typical numerical aperture ratios described above, substantially all
of the collected light is coupled into the inner cladding (e.g., 242) and
only a small fraction is coupled into the fiber core (e.g., 243). For a
typical inner cladding NA=0.47 and a core NA=0.12, about 93% of light
collected by the TX/RX lens (e.g., 250) can be estimated to couple into
the inner cladding (e.g., 242).

[0023] Actually, the fraction of the light power coupled into the inner
cladding (e.g., 242) can be expected to be higher than that calculated
from the NA ratio alone. Since collected light fills the entire lens
aperture, for a lens with non-diffraction limited performance the focused
spot diameter will be significantly larger than the fiber core. This
"spill-over" effect reduces the fraction of received light that couples
into the core, regardless of incidence angles of the focused rays. The
lens surface shape can be intentionally designed so that it performs near
the diffraction limit over a small fraction of its diameter but operates
far from diffraction limit over the remaining surface. For example, a
lens with a numerical aperture of 0.5 can be made to produce
near-diffraction limited beam collimation for light emerging from DCF
core with an NA=0.12, but produce a focused spot that is significantly
larger than diffraction limit for collected light filling the entire lens
surface corresponding to an NA of 0.5.

[0024] An integral aspect of the disclosure is the diplexer (e.g., 130).
The variously disclosed diplexer performs the function of separating the
received light (e.g., 121) from the transmit light (e.g., 111) that is
propagating in the opposite direction. Ideally, the diplexer should
introduce minimal loss for both the received light and transmitter light:
its construction should be compact, insensitive to vibrations and
temperature variations, and compatible with fiber coupled light sources
and the double cladding fiber-based monostatic TX/RX shown in FIG. 2.

[0025] The first exemplary diplexer configuration is shown in FIG. 3a.
FIG. 3b shows a cross-sectional view of an exemplary coupling region of a
diplexer configuration based on such a fiber coupler.

[0026] Such an exemplary all-fiber device can be made with two fiber
types, one with a DCF structure (e.g., 340) and the other with a
single-cladding/multimode-core (MM) structure (e.g., 330). The two fibers
are fused together or are in optical contact with each other in order to
allow optical power transfer between the inner cladding of the
double-cladding fiber (e.g., 340) and the MM fiber (e.g., 330). The outer
cladding of the DCF (e.g., 340) and jacket of the MM fiber (e.g., 330)
are stripped away in the coupling region (e.g., 331) to allow optical
contacting or fusion of the inner cladding of the double-cladding fiber
(e.g., 340) and the MM fiber (e.g., 330). The coupler (e.g., 300) is
fabricated so that strong optical coupling (e.g., 331) is achieved
between the inner cladding of the DCF (e.g., 340) and the MM fiber (e.g.,
330), without causing significant perturbation of the signal light
propagating in the DCF core (e.g., 343). This assures that there is no
significant light loss for the transmitter light coupled from the optical
source (e.g., 310, such as a fiber MOPA) into the core (e.g., 343) of the
double-cladding fiber (e.g., 340). The MM fiber output is coupled into
the receiver photodiode (e.g., 320).

[0027] For a sufficiently long coupling length, the fraction of power
propagating in the multimode DCF inner cladding (e.g., 340) that
cross-couples into the MM single cladding fiber (e.g., 330) is given by,

AM/(ADCF+AM)

[0028] where ADCF is the cross-sectional area of the DCF inner
cladding and AM is the cross-section area of the MM fiber. For a 200
μm diameter of MM fiber and 100 μm DCF inner cladding diameter,
this coupling fraction is 80%. While the 20% loss in the received signal
is acceptably low for most LIDAR and LRF systems, a larger coupling
fraction is often desirable and can be achieved by several techniques.
For the first exemplary embodiment, the diameter of the MM fiber (e.g.,
330) can be increased, although this is not always desirable since the
output of MM fiber output is coupled into a small diameter photodiode
(e.g., 320), with a typical active area diameter of 50-80 μm required
for frequency a response of ˜1 GHz. For the second exemplary
embodiment, the inner cladding of the DCF (e.g., 340) can be reduced to
achieve higher cross-coupling ratio, although this is also not always
desirable since a smaller DCF reduces the FOV of the monostatic TX/RX,
and also makes the fiber more difficult to work with.

[0029] A third exemplary embodiment is shown in FIG. 4. In order to
increase the cross-coupling fraction, the exemplary configuration shown
in FIG. 4 uses a modified coupler structure that circumvents these
limitations. As in the previously described coupler, the coupler 400 in
FIG. 4 can be comprised of a DCF fiber (e.g., 440) that is fused to, or
is in optical contact with, a single cladding MM fiber (e.g., 430). In
this exemplary modified coupler, however, the DCF (e.g., 440) is
gradually tapered down to a sufficiently small diameter to achieve a
desired cross-coupling ratio. Outside the cross-coupling section, the DCF
(e.g., 440) can have a larger diameter for easy handling and to maintain
large FOV for the receiver (e.g., 420). The coupler (400) in FIG. 4 can
have distinct sections, e.g., 4 sections as labeled A,B,C,D. As
exemplified, coupling between fibers occurs in sections A-C. Section A
can have an un-tapered length of DCF (e.g., 440) in order to allow high
NA rays in the DCF inner cladding to couple into the MM fiber (e.g., 430)
before reaching the down-taper. This is required since the down-taper
increases the NA of the inner cladding light by the taper ratio, or the
ratio of inner cladding diameter at the wide end of the taper to that at
the narrow end. Section A assures that light propagating through the DCF
taper does not exceed the NA of the DCF inner cladding. The length of
coupling section B has a DCF down-taper, and section C also has a
down-tapered DCF to achieve maximum DCF-MM fiber cross-coupling allowed
by the fiber cross-sectional area ratio. In section D the down-tapered
DCF is separated from the MM fiber before reaching the up-taper. This
coupler feature is required to prevent light that is already in the MM
fiber (e.g., 430) from coupling back into the DCF (e.g., 440).

[0030] To avoid losses for the DCF core (e.g., 443) in the coupler 400
shown in FIG. 4, the taper fraction has to be sufficiently small so that
all modes that are launched into it from the light source (e.g., 410)
remain well confined in the core (e.g., 443). For a typical single mode
core with a typical diameter of 10 μm and NA=0.12, this means that the
taper ratio should be below approximately 2.0.

[0031] Another exemplary embodiment of a diplexer, as shown in FIG. 5,
uses micro-lenses. In this exemplary arrangement the transmitter light
emerging from the fiber core (e.g., 513) is coupled into the core (e.g.,
543) of a DCF fiber (e.g., 540) using a pair of lenses 1 and 2. The
lenses can be arranged in an imaging configuration, where the near-field
intensity distribution from the core (e.g., 513) of the fiber connected
to the source (e.g., 510) is first imaged by lens 1 at half-point between
the two lenses, where it passes through a hole in an angled mirror (e.g.;
530). The transmitted light is then imaged onto the face (e.g., 544) of
the DCF 540 by lens 2. If the cores (e.g., 513 & 543) of the two fibers
are identical, then nearly complete (discounting Fresnel losses)
core-to-core coupling efficiency can be achieved with such 1:1 imaging
magnification. For dissimilar core diameters other magnification factors
can be used to match the mode field diameters of the two fiber cores.

[0032] The received light emerging from the inner cladding (e.g., 542) of
the DCF is imaged onto the face of a MM photo-receiver fiber (e.g., 521)
by the lens pair 2, 3. An image of the near-field distribution of the DCF
is imaged onto the angled mirror (e.g., 530) by lens 2, and after
reflection is re-imaged onto the face (e.g., 523) of the MM fiber by lens
3. The MM fiber diameter, DCF inner cladding diameter and magnification
factor of the lens pair 2, 3 are chosen so that the image of the DCF
inner cladding matches the MM fiber diameter. A small fraction of the
received light is lost because it falls on the hole in the diplexer
mirror (e.g., 530). This fraction is given by the ratio between the core
and inner cladding areas Acore/(ADCF), which for a 10 μm
core and a 100 μm cladding corresponds to a loss of only 1%.

[0033] It is obvious that many modifications and variations of the present
invention are possible in light of the above teachings. It is therefore
to be understood that within the scope of the appended claims, the
invention may be practiced otherwise than as described.

Patent applications by Lew Goldberg, Fairfax, VA US

Patent applications by UNITED STATES OF AMERICA, AS REPRESENTED BY THE SECRETARY OF THE ARMY